The fabrication of lenses with a radial or a spherical gradient geometry has been a problem of considerable interest. Several methods may be considered to fabricate lens blanks with a pure radial gradient. However, such methods invariably involve a process limitation which makes achieving a gradient with the desired design profile and of large (macro) size nearly impractical.
The following definitions are provided as an aid to understanding the invention:
Axial Gradient:
The index is constant in planes orthogonal to the optical axis of the lens, which is denoted as the z-axis, but varies as one moves along the z-axis. The index is a function of z only.
Spherical Gradient:
The index is constant on spherical surfaces that surround a central point. If the lens is cut out of such an index distribution, it can be thought of as an off-center (thin) slice out of an onion. The index is a function of the distance from the center R, where R.sup.2 =z.sup.2 +x.sup.2 +y.sup.2.
Generalized Spherical Gradient:
The surfaces of constant index are surfaces of revolution; they are formed by rotating parabolas, hyperbolas, or any other smooth curve around the optical axis. Rotating a circle yields a spherical surface.
Radial Gradient:
The index is constant along the optical axis of the lens, but varies as a function of the two-dimensional transverse distance from the optical axis. The surfaces of constant index form tubes, with the axis at their center. The index is in-dependent of z, but is a function of the distance r from the z-axis, where r.sup.2 =x.sup.2 +y.sup.2.
Generalized Cylindrical Gradient:
The surfaces of constant index are surfaces of translation; they are formed by translating arcs of circles, parabolas, hyperbolas, or any other smooth curve along an axis perpendicular to the optical axis. Translating a circle yields a tube-like surface. The index is a function of the z and the transverse distance x. The index is independent of y, the direction of translation.
The term "shaped gradient" will be employed herein for conciseness to describe all the foregoing gradients, and includes the combination of radial (two-dimensional transverse) plus axial, which results in a spherical or generalized spherical gradient, and the combination of one-dimensional transverse plus axial, which results in a cylindrical or generalized cylindrical gradient. Further, any arbitrary shape of the gradient may be employed, such as corrugations, dimples, saddles, etc.
The fabrication of lenses with axial gradients is known; see, e.g., U.S. Pat. No. 5,044,737, issued Sep. 3, 1991, and assigned to the predecessor of the present assignee. In that reference, a discussion of So-called "micro" processes is given. Such micro processes include diffusion into a lens blank of a refractive index-altering element of differing atomic number by immersion in a molten salt bath of, e.g., silver chloride or by coating the lens blank with a thin layer that has a different index of refraction and then heating. Other techniques include implantation of ions into the surface of the lens blank and chemical vapor deposition of a species onto the surface thereof.
All such micro processes suffer from the fact that a substantial period of time is required to produce a gradient in the index of refraction, and that in any event, the gradient is invariably Gaussian or a variation thereof and the processing results in a gradient only near the surface. Gradients throughout relatively thick lens blanks (thickness greater than about fifteen millimeters) are not achievable in reasonable periods of time for commercial applications. Further, the maximum practically achievable index change is on the order of 0.05 for these micro lenses. This limits the use of these gradient index of refraction (GRIN) lenses.
Recently, so-called "macro" processes have been developed. By macro process is meant the use of bulk glass processing techniques, which can result in considerably thicker glass blanks having a gradient in index of refraction that varies substantially continuously through the entire lens blank. Examples of such processing includes fusing at least two layers of frits or plates together (each layer having a slightly different index of refraction). Typically, the top and bottom members are each of separate composition, with the intermediate members, if present, mixtures of the two end compositions to give a desired gradient profile or glasses of intermediate composition between the top and bottom members. Examples of such processing techniques are described in U.S. Pat. Nos. 4,883,522, 4,907,864, and 4,929,065, each assigned to the predecessor of the present assignee.
The macro processes yield lens blanks which have greater thicknesses than obtained by micro processes; dimensions of nearly 50 mm with a gradient along the entire thickness axis are easily fabricated. Further, differences in the index of refraction from one side to the other of 0.1 to 0.25 are routinely achievable, with differences approaching 0.5 attainable.
The macro processes permit fabrication of lens blanks with design parameters not heretofore available to the lens and optical designer. For example, it is desired to correct not only spherical aberrations, but also chromatic aberrations. Further, it is desired to reduce the effects of temperature on the optical properties.
Shaped gradient lenses permit new designs and arrangements of optical elements, thereby forming optical systems with desirable properties. The spherical and chromatic properties of a shaped gradient lens are different from both a homogeneous lens and a lens with a purely axial gradient profile. A shaped lens blank with a combination of a radial and an axial index profile will extend the options of optical designers, allow better design compromises, and lead to more efficient optical instruments. Consequently, efforts continue to develop processes for fabricating shaped gradients in lens blanks.